1. Field of the Invention
The present invention relates to a vacuum brake booster which reduces the force required for operating a pedal-operated brake system of vehicles and, more particularly, to a control valve mechanism which is operated by a push rod linked to the brake pedal which controls a pressure differential between front and rear chambers in the housing of the booster.
2. Description of the Related Art
A prior type vacuum brake booster for use in vehicles is described, for example, in Japanese Utility Model Laid-Open Publication No. 57-92569 published on Jun. 7, 1982.
The vacuum brake booster includes a power piston which divides an interior of a body into a variable pressure chamber and a constant pressure chamber constantly applying a negative pressure and a control valve mechanism for controlling a pressure differential between the variable pressure chamber and the constant pressure chamber in the body. The control valve mechanism includes a valve plunger slidably received in a power piston which is linked to the brake pedal via a push rod and a control valve element secured to the power piston at one end portion thereof and having a seal surface at its other end portion. The seal surface of the control valve element is normally urged by a first coil spring disposed in the power piston and which is in contact with a seat surface of the end portion of the valve plunger. The control valve element is secured in an air tight manner to the cylindrical portion of the power piston at its outer circumference portion by a retainer engaging a second coil spring which urges the push rod toward the brake pedal. The control valve element is provided with a rolling portion which is flexible in the axial direction so as to force the seal surface to follow the movement of the push rod and includes a barrel-shaped portion and a flange portion normally subject to atmospheric pressure and negative pressure (vacuum) at the inner and outer surfaces, respectively.
In this prior arrangement, when the brake pedal is depressed, the control valve element and the valve plunger are linked to the brake pedal by the first coil spring against the second coil spring, and the seal surface of the control valve element is contacted by a valve seat of the power piston and the communication between the variable pressure chamber and the constant pressure chamber is interrupted. Thereafter, the seat surface of the end portion of the valve plunger is separated or removed from the seal surface of the valve plunger, whereupon atmospheric pressure is introduced into the variable pressure chamber and a pressure differential is produced between the variable pressure chamber and the constant pressure chamber so that the desired operational pressure force is produced by the push rod.
In this prior arrangement, the retainer is extended along the inner direction to the inner surface of the flange portion of the rolling portion. An inner circumference edge portion of the retainer is extended up to about the middle portion of the flange portion in order to ensure the performance of the flexibility so as to be able to follow to the movement of the push rod. However, the atmospheric pressure and the negative pressure are normally applied to the inner and outer surface of the circular areas from about the middle portion to an inner circumference edge portion of the flange portion, respectively. Thereby, the control valve element is urged toward the valve seat of the power piston by the pressure differential between the atmospheric pressure and the negative pressure. Since it is necessary to separate the control valve element with the push rod from the seat surface of the power piston against the pressure differential when the brake pedal is not depressed in order to control the braking force in response to the brake pedal, it is necessary to enlarge the predetermined load of the second coil spring as a second urging force. Consequently, since the depressing force of the brake pedal is increased in the first stage of the depression of the brake pedal and the input load of the vacuum booster is increased, the prior arrangement has a drawback in that it produces an adverse influence on the operational feel of the brake operation.